This invention relates to a tone signal generation device and, more particularly, to a device capable of performing an interpolation between amplitudes of tone waveshape sample points in accordance with a tone range of a tone to be generated. The invention relates also to a tone signal generation device in which waveshape resolution has been improved by interpolating amplitudes of tone waveshape sample points and inharmonic noise has been removed by having the sampling frequency synchronized with the pitch of the tone.
A typical method for generating a tone waveshape sample point amplitude signal on the basis of phase information which changes with the pitch of a tone to be generated is to store a tone waveshape divided in a specific number of sample points in a memory and have this waveshape read out in response to phase information. If reading of tone waveshapes of the same note name in different octaves is considered, the amount of phase change per one sampling time becomes larger as the octave is higher and smaller as the octave is lower. For this reason, in reading out a tone waveshape of a relatively low tone range, it happens that an amplitude value of the same sample point is repeatedly read out over plural sampling times. This means that an effective sampling frequency decreases by an amount corresponding to the same sample point amplitude value which has been repeatedly read out. If, for example, different sample point amplitude values are read out in response to a sampling clock pulse of a frequency fs, the effective sampling frequency in the read out tone waveshape signal is fs but if the same sample point amplitude value is read out twice in succession, the effective sampling frequency of the read out tone waveshape signal decreases to fs/2. The term "effective sampling frequency" herein means a frequency at which the sample point amplitude value actually changes in an obtained tone waveshape signal.
In a case where the effective sampling frequency has decreased as described above, the actual sampling frequency decreases no matter how high the nominal sampling frequency (i.e., frequency of the sampling clock pulse) may be and, accordingly, a tone range in which an aliasing noise occurs becomes lower with an increasing tendency to generation of such aliasing noise. This is not a sole problem of the above described system of the waveshape memory accessing type but can take place in any tone waveshape sample point amplitude signal generation system.
On the other hand, known in the art is a system according to which sampling of a higher resolution can be realized by interpolating respective sample points of a tone waveshape sample point amplitude signal which has once been generated. For example, U.S. Pat. No. 4,036,096 discloses a system according to which finer interpolation between sample points is effected as the tone range of a tone to be generated becomes higher whereby a tone waveshape in a higher tone range can be made a smooth waveshape with little harmonic content and an aliasing noise in the higher tone range can be eliminated. It was not an object of this prior art system, however, to solve the problem of decreasing in the effective sampling frequency in a lower tone range and resulting generation of an aliasing noise in the lower tone range and so the prior art system cannot solve this problem at all.
In an electronic musical instrument of a digital processing type, a tone waveshape is synthesized by sampling tone waveshape amplitudes at a predetermined sampling interval. For synthesizing a tone waveshape by sampling, there have been practiced two methods. One is to perform sampling always at a constant sampling frequency regardless of the frequency of a tone to be synthesized and the other is to have the sampling frequency synchronized with the frequency of a tone to be synthesized. In the former, since the ratio between a tone frequency and a sampling frequency is a noninteger ratio, an aliasing noise which is not harmonic to the tone frequency is generated as will be apparent from the sampling theorem. In the latter, the tone frequency (pitch) is harmonized with the sampling frequency so that a component which is generated by aliasing is harmonized with the tone frequency and thereby is prevented from becoming a noise. Japanese Preliminary Patent Publication No. 171395/1982 discloses an art in which, in a system generating tone signals in plural channels on a time shared basis, the pitch of a tone signal generated in each channel is caused to synchronize with the sampling frequency. The above mentioned U.S. Pat. No. 4,036,096 discloses a system in which waveshape resolution is improved by interpolating amplitudes of sample points of a generated waveshape signal.
The system interpolating sample point amplitude values of a tone waveshape signal is advantageous in that a quality of a generated tone can be improved with a relatively simple construction. The interpolation performed in the prior art interpolation technique, however, is not synchronized with the pitch of a tone and there arises the problem that a component of interpolation operation timing becomes a noise with respect to a generated tone.